1. Field of the Invention
This invention relates to amplification techniques based on the Raman effect and was developed with particular reference to the need of developing broadband and high-power laser sources which can be used in Raman amplifiers for amplifying optical signals on fibers.
2. Description of Background Art
In fiber optic transmission systems, the continuous growth in transmission bandwidth requirements has pushed research activities in two main directions: increasing the signal frequency (bit rate) and increasing the number of channels adapted to be multiplexed in WDM and DWDM systems.
In this scenario, Raman amplification techniques (both distributed and discrete) play an increasingly important role in overcoming the intrinsic limitations of traditional systems based, for example, on Erbium Doped Fiber Amplifiers (EDFA). In principle, this also refers to the aspects related to bandwidth and noise. Additionally, Raman amplification can be useful for eliminating or compensating EDFA gain irregularities (tilting and/or ripples) due to the presence of optical amplifiers (EDFA) in the transmission line.
For a general overview of Raman amplification techniques, useful reference can be made to the following works: Alan Evans, “Raman Amplification in Broadband WDM Systems,” OFC 2001, TuF4-1; M. D. Marmelstein, et al., “A High Efficiency Power-Stabile Three-Wavelength Configurable Raman Fiber Laser,” OFC 2001, PD3-1; Do Il Chang, et al., “Dual-Wavelength Cascaded Raman Fiber Laser,” OFC 2001, MA6-1.
The frequency/wavelength range in which the amplification effect is attained is identified in principle by the frequency of the source used as a pump. Consequently, in order to extend the frequency range for Raman amplification and to make the amplification action more regular in this frequency range, pumping with several sources working at different wavelengths can be resorted to. Each source consequently generates radiation at a wavelength suitable for producing a Raman gain in a different region of the electromagnetic spectrum. To fulfill the above requirement, it is mandatory to pump the medium in which the Raman effect is attained (hereinafter briefly called the “Raman medium”) by using several pump wavelengths with suitable power levels. The objective is to approximate as closely as possible the ideal model of a single high-power source capable of generating a broadband output signal and preferably offering the possibility of implementing reliable control, both of the output power and of the wavelength in each frequency range concerned.
With reference to this, techniques based on the solution of wavelength and polarisation multiplexing of the outputs of several low-power pump laser diodes were recently proposed to provide Raman gain over an adequately broad bandwidth.
These solutions are documented, for example, in the works by Y. Emori, et al., “1-THz-Spaced Multi-Wavelength Pumping for Broadband Raman Amplifiers,” ECOC 2000, Dienstag 4.4.2 and by Y. Emori, S. Namiki, “1000 nm Bandwidth Flat Gain Raman Amplifiers Pumped and Gain-Equalised by 12-Wavelength Channel WDM High-Power Laser Diodes,” OFC 1999, Pd19-1.
These solutions are essentially based on the principle of multiplexing a sufficiently high number of narrowband sources, i.e. sources whose output spectrum, in the wavelength range, has a width which is typically lower than one nanometre. The main drawbacks of these solutions are essentially related to system reliability (penalised by the need of using a high number of sources and respective coupling components) and to the fact that output power is however rather low, also due to intrinsic losses related to multiplexing operations of the radiation generated by the single sources.
Noise of the pump sources is another crucial factor which must be considered in Raman fiber amplifier design. This fact (described, for example, in the work by C. R. S. Fludger, et al., “Pump to Signal RIN Transfer in Raman Fiber Amplifiers”, El. Lett. Vol. 37, No. 1, pg. 15-17, Jan. 4, 2001) along with cost and dimension factors penalises both the solutions described above and other recently proposed solutions based on multiplexing two or more Raman fiber lasers.